The present invention is generally directed to a dual layer photoresist system which is particularly applicable to the formation of high resolution conductive patterns on insulative substrates exhibiting surface roughness and/or non-planar surface variations. More particularly, the present invention is directed to a two level resist configuration and method for photopatterning which employs a thin layer of ablatable photoabsorptive polymer over which is disposed a thicker layer of substantially transparent material which is exploded away during laser ablation of the underlying photoabsorptive layer.
It is nearly impossible to photopattern surfaces exhibiting surface irregularities and/or non-planar aspects, by normal resist methods. Some surface irregularities may occur as a result of the desire to employ filler material such as glass fibers for added strength. Other surface irregularities may occur as a direct result of structural design features incorporated into a molded part. Such features include channels, ridges and bosses, for example. For such workpieces, the use of resist laminates, spin coatings and the like prevent proper resist thickness control and contact maintenance so as to render such resists and processes non-viable.
Moreover, parts patterned using conventional photoresists such as those known in the semiconductor patterning arts are generally fine tuned for use with medium to high pressure mercury arc lamps and do not perform well, if at all, in the presence of collimated, single frequency component radiation such as that produced by lasers, especially ultraviolet lasers. With regard to these conventional resists, a problem exists in that the laser energy is absorbed in the top layer, and once cured, this layer then blinds the surface beneath it, thus rendering the curing of this subsurface material extremely difficult, if not impossible. The resist system and process of the present invention solve these problems and others as is more particularly described below.
In particular, the resist configuration and method of the present invention is directly applicable to the patterning of three-dimensional electronic circuit boards, modules and the like. Additionally, the present invention is also applicable to patterning three dimensional circuit boards and/or surfaces which also include feed-through apertures. As used herein, and in the appended claims, the use of the phrase "three-dimensional" refers to surfaces which are rough, either because of a molding process or because of the use of filler material. This phrase also refers herein to surfaces which exhibit structural design features such as channels, ridges, bosses and the like.
The concept of patterning such three-dimensional boards with a mask with the use of normal light sources is almost impossible. The 1.degree. to 3.degree. diffractions in the light sources, plus lens irregularities make non-contact mask use extremely difficult due to the undercutting of the patterns. Contact mask processes are virtually impossible due to the three-dimensional nature of the surface. Pattern compensations can aid in this problem but edge definition is nonetheless reduced. Furthermore, the use of a standard light source with its associated long exposure time over large areas is prohibitive. For example, it takes minutes to expose large areas with a fixed 500 watt or 1 kilowatt light source.
Points, tips, bosses, etc. of three-dimensional boards cannot easily be coated with resist material in any way. Even sprayed on resists do not work well for this application. Furthermore, films of resist, such as RISTON.TM., cannot be roll laminated to such three-dimensional boards. Additionally, there is no easily procurable film resist having a thickness less than approximately 0.7 mils (that is, about 18 microns). In addition, there is none that lasers can expose correctly. In most of the cases of interest herein, a resist thickness of 2 to 3 mils (50 to 75 microns) is needed to cover the surface finish alone. No normal resist is known which can work at these thicknesses with a device such as an excimer laser, for example. Furthermore, the concept of disposing a thin resist which is supported by a clear continuous carrier, such as RISTON.TM., which is removed before developing to give the appearance of a thick resist also does not work since the resist layer is confused with the over-layer due to their mutual solubility with respect to one another.
As indicated above, a further area of concern is that the percentage of filler in a polymer substrate, such as one comprising a polyetherimide such as ULTEM.TM., makes coating difficult. This aspect plus variations resulting from internal mold finish, make product surface finish variable. Various "pretreats" are, however, employed to promote adhesion. All of the fillers nonetheless produce a very rough surface. Normal resists cannot coat such surfaces, since resists are non-conformable to points, edges, etc. yielding voided areas.
In order to achieve fine line and spacing, one needs a well collimated light source, a good mask and a good, thin resist. There is a direct relationship between the aspect ratio associated with resist thickness and the ability to resolve line and area details. More particularly, the thinner the resist, the greater the ability to resolve fine lines. However, the thicker the resist, the more accurate and precise the radiation source must be. Moreover, the radiation source must exhibit an areal energy density sufficiently high to effect the desired changes in the resist (here ablation). The frequency of the radiation source must also be such that the resist is absorptive at that frequency. Additionally, the mask employed should be able to withstand laser radiation bombardment at the required frequencies without degration in its structure or pattern.
Photoresists that are conventionally used in the semiconductor arts are so absorptive at excimer laser frequencies and energies, that proper exposure is not feasible. Additionally, positive resists are desired whenever through-holes are present. More particularly, if negative resist material dries in the through-holes, such holes cannot be exposed with the laser. This is because the holes are typically approximately 0.1 inches (100 mils) deep and the absorption is 99.99% in the first micron (0.04 mils). In processing, the resist comes out and allows the plated through holes to be destroyed in the etching step. However, positive resists are so-called chain break resists and do not exhibit this problem.
All of the aforementioned problems have made it nearly impossible to make production level three-dimensional parts possessing fine lines and feed-through apertures.